EP3835448B1 - Stahl für druckbehälter mit ausgezeichneter oberflächenqualität und schlagzähigkeit sowie verfahren zu seiner herstellung - Google Patents

Stahl für druckbehälter mit ausgezeichneter oberflächenqualität und schlagzähigkeit sowie verfahren zu seiner herstellung Download PDF

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EP3835448B1
EP3835448B1 EP19847417.3A EP19847417A EP3835448B1 EP 3835448 B1 EP3835448 B1 EP 3835448B1 EP 19847417 A EP19847417 A EP 19847417A EP 3835448 B1 EP3835448 B1 EP 3835448B1
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steel material
less
temperature
excluding
steel
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EP3835448A1 (de
EP3835448C0 (de
EP3835448A4 (de
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Tae-Il SO
Dae-Woo Kim
Sang-Deok Kang
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Posco Holdings Inc
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Posco Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0294—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a localised treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Definitions

  • the present invention relates to a steel material for petrochemical production equipment, storage tanks, and the like, and more particularly, to a steel material for a pressure vessel having excellent surface quality and impact toughness and a method for manufacturing same.
  • a post weld heat treatment is performed to prevent deformation of a structure after the welding, to stabilize a shape and a size, and to relieve stress generated during the welding.
  • PWHT post weld heat treatment
  • Such a post weld heat treatment is performed for a lengthy period to cause softening and growth of a steel sheet and coarseness of a carbonitride, so that strength and impact toughness are significantly deteriorated.
  • a base metal decreases in strength and tends to increase in ductile-to-brittle transition temperature (DBTT) through processes such as carbon re-diffusion, potential recovery, grain growth and carbide growth and precipitation, and the like, when a PWHT is performed for a lengthy period.
  • DBTT ductile-to-brittle transition temperature
  • Patent Document 1 discloses a steel material having strength and toughness which are not reduced by forming a ferrite-bainite dual-phase microstructure and controlling a packet size of the bainite and size and amount of a carbonitride even after a PWHT is performed for a lengthy period.
  • Patent Document 1 a large amount of copper (Cu) is contained in an alloy composition to result in high sensitivity to occurrence of surface defects.
  • a large amount of titanium (Ti) and a large amount of boron (B) are contained in the alloy composition, so that a value of low-temperature impact toughness is significantly decreased due to a coarse carbonitride.
  • Heat treated high tensile strength steel is for example described in JP H10 102194 A , JP H10 102193 A , WO 2017/111290 A1 and WO 2017/105107 A1 .
  • An aspect of the present invention is to provide a steel material for a pressure vessel having excellent surface quality while having excellent strength and toughness even after a post weld heat treatment (PWHT) is performed for a lengthy period, and a method of manufacturing the steel material.
  • PWHT post weld heat treatment
  • a steel material for a pressure vessel having excellent surface quality and impact toughness includes, by weight percentage (wt%), 0.1 to 0.15% of carbon (C), 0.15 to 0.5% of silicon (Si), 1.2 to 1.8% of manganese (Mn), 0.01% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01 to 0.05% of aluminum (Al), 0.01 to 0.05% of niobium (Nb), 0.01 to 0.25% of nickel (Ni), 0.1% or less (excluding 0%) of copper (Cu), 0.01 to 0.1% of molybdenum (Mo), 0.01 to 0.05% of vanadium (V), 0.003% or less (excluding 0%) of titanium (Ti), 5 ppm or less (excluding 0 ppm) of boron (B), 20 to 100 ppm of nitrogen (N), and a balance of iron (Fe) and other inevitable impurities, and satisfying Relational Expression 1 below.
  • the steel material includes a composite structure of ferrite and bainitic ferrite as a microstructure. 0.5 ⁇ Ti + Nb + B / 3.5 N + Cu/Ni ⁇ 2.5 where each element refers to a content by wt%.
  • the steel material comprises ferrite having an area fraction of 40 to 80% and retained bainitic ferrite as a microstructure, and wherein when a thickness of the steel material is 35 mm or less, the steel material comprises ferrite having an area fraction of 20 to 60%, tempered martensite having an area fraction of 20% or less (including 0%), and retained bainitic ferrite as a microstructure, in a 1/4t point (where t refers to a thickness (mm) of the steel material), the tensile strength is 480 MPa or more, and Charpy impact absorption energy at a temperature of -50°C is 150J or more.
  • tensile strength is 480 MPa or more
  • Charpy impact absorption energy at a temperature of -50°C is 100J or more.
  • the steel material has a thickness of 10 to 150 mm.
  • a method of manufacturing a steel material for a pressure vessel having excellent surface quality and impact toughness includes : reheating a steel slab, having the alloy composition above and satisfying Relational Expression 1 above, to a temperature of 1050 to 1200°C, rough rolling the reheated slab within a temperature range of Ar3+100°C to 1200°C; finish hot rolling the rough-rolled slab at a temperature of Ar3+30°C or more to manufacture a hot-rolled steel sheet; air cooling the hot-rolled steel sheet to room temperature; reheating the air-cooled hot-rolled steel sheet to a temperature of Ac3 or higher, and then maintaining the reheated hot-rolled steel sheet for (1.3t+20) minutes or more (where t refers to a thickness (mm) of the steel material); cooling the hot-rolled steel sheet to room temperature at a cooling rate of 2°C/sec or more after the maintaining; performing a tempering process in which the cooled hot-rolled steel sheet is maintained within a temperature range of 600 to 700°C for (
  • a steel material for a pressure vessel having excellent surface quality as well as excellent strength and impact toughness even after a post weld heat treatment (PWHT) is performed, may be provided.
  • PWHT post weld heat treatment
  • a steel material of the present invention may be appropriately applied to petrochemical production equipment, storage tanks, and the like.
  • the present inventors have conducted deep research into a steel material for a pressure vessel having improved surface quality as well as excellent strength and toughness even when a post weld heat treatment (PWHT) is performed on the steel material for a pressure vessel for a lengthy period. As a result, it is confirmed that a steel material, capable of securing target strength and tough even after a post weld heat treatment (PWHT) is performed at high temperature for a lengthy period and significantly reducing defects such as star cracking in a surface, may be provided by optimizing an alloy composition and manufacturing conditions. Based on this knowledge, the inventors have implemented the present invention.
  • PWHT post weld heat treatment
  • the present inventors have found that it is necessary to control addition and composition of alloying elements so as to reduce sensitivity to occurrence of surface defects during a process of manufacturing a steel material for a pressure vessel.
  • Cu copper
  • Ni nickel
  • a steel material for a pressure vessel having excellent surface quality and impact toughness includes, by weight percentage (wt%), in detail, 0.1 to 0.15% of carbon (C), 0.15 to 0.5% of silicon (Si), 1.2 to 1.8% of manganese (Mn), 0.01% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01 to 0.05% of aluminum (Al), 0.01 to 0.05% of niobium (Nb), 0.01 to 0.25% of nickel (Ni), 0.1% or less (excluding 0%) of copper (Cu), 0.01 to 0.1% of molybdenum (Mo), 0.01 to 0.05% of vanadium (V), 0.003% or less (excluding 0%) of titanium (Ti), 5 ppm or less (excluding 0 ppm) of boron (B), and 20 to 100 ppm of nitrogen (N) .
  • Carbon (C) is an element effective in improving the strength of steel, and C is included in an amount of, in detail, 0.01% or more to secure sufficient strength. However, when the content of C is greater than 0.15%, low-temperature impact toughness of a base metal and a welding zone may be significantly reduced.
  • the content of C is, in detail, 0.1 to 0.15%, and, preferably, 0.12 to 0.14%.
  • Silicon (Si) is used as a deoxidizer, and is an element advantageous for improving strength and toughness.
  • Si is an element advantageous for improving strength and toughness.
  • the content of Si is greater than 0.5%, low-temperature toughness may be decreased and weldability may be deteriorated.
  • the content of Si is less than 0.15%, a deoxidation effect may be insufficient.
  • the content of Si is, in detail, 0.15 to 0.5%.
  • Manganese (Mn) is an element advantageous for securing strength due to a solid solution strengthening effect.
  • the content of Mn is, in detail, 1.2% or more.
  • Mn may combine with S in the steel to form MnS, causing room-temperature elongation and low-temperature toughness to be significantly reduced.
  • the content of Mn is, in detail, 1.2 to 1.8%. More advantageously, Mn may be included in an amount of 1.3 to 1.7%.
  • Phosphorus (P) is an element advantageous for securing strength and corrosion resistance of steel.
  • impact toughness may be significantly reduced. Accordingly, the content of P may be maintained as low as possible.
  • the content of P is controlled to be 0.01% or less. More advantageously, the content of P may be controlled to be 0.008% or less. However, 0% is excluded in consideration of an inevitably added level of P.
  • S is an impurity present in steel.
  • S may combine with Mn to form a non-metallic inclusions (for example, MnS), causing impact toughness of the steel to be significantly reduced.
  • the content of S is, in detail, 0.01% or less. More advantageously, the content of S may be controlled to be 0.003% or less.
  • Aluminum (Al) is an element added to deoxidize molten steel, and is included in an amount of, in detail, 0.01% or more to sufficiently obtain the deoxidation effect. However, when the content of Al is greater than 0.05%, nozzle clogging may occur during continuous casting.
  • the content of Al is, in detail, 0.01 to 0.05%.
  • Niobium (Nb) precipitates in the form of NbC or Nb(C,N) to significantly improve strength of a base metal and a welding zone.
  • Nb solid-solubilized during high-temperature reheating may inhibit recrystallization of austenite and transformation of ferrite or bainite to refine a structure.
  • Nb may improve stability of austenite to promote formation of a hard phase such as martensite or bainite even in cooling at a low rate. Therefore, Nb is useful in securing the strength of the base metal.
  • Nb is included in an amount of, in detail, 0.01% or more.
  • Nb is an expensive element and, when a significantly large amount of Nb is added together with Ti, coarse (Ti, Nb) (C, N) may be formed during a heating process or after a PWHT to reduce low-temperature impact toughness.
  • the content of Nb is, in detail, 0.01 to 0.05%.
  • Nickel (Ni) is an element, capable of simultaneously improving strength and toughness of the base material. To obtain such an effect, Ni is included in an amount of, in detail, 0.01% or more. Since Ni is an expensive element, it may be economically disadvantageous to include Ni in an amount of greater than 0.25%.
  • the content of Ni is, in detail, 0.01 to 0.25%.
  • Copper (Cu) is an element, capable of improving strength while significantly reducing a decrease in the toughness of the base material.
  • Cu is included in an amount of, in detail, 0.1% or less (excluding 0%).
  • Molybdenum has an effect of inducing formation of a bainite or martensite phase while significantly improving hardenability to inhibit formation of ferrite even when a small amount of Mo is added.
  • Mo is advantageous for significantly improving the strength of the steel.
  • Mo may be included in an amount of, in detail, 0.01% or more. However, when the content of Mo is greater than 0.1%, there is a high possibility that hardness of a welding zone may be significantly increased to reduce toughness.
  • Mo is included in an amount of, in detail, 0.01 to 0.1%. More advantageously, Mo may be included in an amount of 0.05 to 0.08%.
  • V Vanadium (V): 0.01 to 0.05%
  • Vanadium (V) is solid-solubilized at a lower temperature than other alloying elements, and may precipitate in a heat-effected zone (HAZ) to prevent a decrease in strength. Accordingly, when the strength is insufficiently secured after a post weld heat treatment (PWHT), V is included in an amount of 0.01% or more. However, when the content of V is greater than 0.05%, a fraction of a hard phase such as an MA phase may be significantly increased to deteriorate low-temperature impact toughness of a welding zone.
  • PWHT post weld heat treatment
  • V is added in an amount of 0.01 to 0.05%.
  • Titanium (Ti) is an element added to reduce occurrence of surface cracking caused by the formation of AlN precipitates .
  • a coarse (Ti,Nb) (C,N) carbonitride may be formed during reheating, tempering or post weld heat treatment (PWHT) to reduce low-temperature impact toughness.
  • the content of Ti is, in detail, 0.003% or less (excluding 0%).
  • B Boron
  • the content of B is 5 ppm or less (excluding 0%).
  • N nitrogen (N) is added together with titanium (Ti)
  • TiN titanium
  • TiN precipitates may be formed to inhibit grain growth caused by welding heat affect.
  • coarse TiN may be formed to reduce low-temperature impact toughness and AlN may be formed to cause surface cracking. Therefore, N is included in an amount of, in detail, less than 100 ppm. To control the amount of N to be less than 20 ppm, a steelmaking load may be significantly increased and a grain growth inhibition effect may be insufficient.
  • the content of N is, in detail, 20 to 100 ppm. More advantageously, N may be included in an amount of 20 to 50 ppm.
  • a balance thereof is iron (Fe) .
  • Fe iron
  • impurities are commonly known to a person skilled in the art, and are thus not specifically mentioned in this specification.
  • the steel material of the present invention having the above-described alloy composition, satisfies a component relationship expressed by the following relationship.
  • Ti, Nb, B, N, Cu, and Ni are elements affecting surface quality and low-temperature toughness of a steel material.
  • the surface quality and the low-temperature toughness may be more advantageously improved by controlling the content of each of the above elements and controlling a content relationship of the above elements by Relational Expression 1.
  • Relational Expression 1 when a value of Relational Expression 1 is 0.5 or less, there is a concern that an AlN precipitate may be formed by excess nitrogen to further deteriorate the surface quality and sufficient strength may not be secured.
  • the value of Relational Expression 1 when the value of Relational Expression 1 is greater than 2.5, a coarse TiNb(C,N) carbonitride may be formed to increase the possibility that low-temperature impact toughness is deteriorated and star cracking occurs in a surface of the steel.
  • the steel material of the present invention includes, in detail, a composite structure of ferrite and bainitic ferrite as a main structure, a microstructure.
  • ferrite refers to acicular ferrite.
  • a fraction of the microstructure may be preferably controlled depending on a thickness of a steel material to be manufactured.
  • a microstructure when a thickness of the steel material is greater than 35 mm, a microstructure includes, in detail, ferrite having an area fraction of 40 to 80% and a balance of bainitic ferrite. Meanwhile, when the thickness of the steel material is 35 mm or less, the microstructure includes, in detail, ferrite having an area fraction of 20 to 60%, tempered martensite having an area fraction of 20% or less (including 0%), and a balance of bainitic ferrite.
  • a ferrite phase When a ferrite phase is insufficiently formed in the microstructure, low-temperature impact toughness may be rapidly deteriorated. Meanwhile, when the ferrite phase is excessively formed, a target level of strength may not be secured. Accordingly, when a composite structure of ferrite and bainitic ferrite is formed as a matrix structure using a microstructure of a steel material provided by the present invention, a fraction of each phase may be preferably controlled to be within the above-described range.
  • the steel material of the present invention may include a carbonitride in a microstructure.
  • the carbonitride is in the form of M(C,N), where M is at least one of Nb, Ti, and V, and a ratio of Nb is, in detail, 20% or more.
  • the above-described carbonitrides have an average particle diameter of, in detail, 100 nm or less. A case, in which the average particle diameter of the carbonitrides is greater than 100 nm, is not preferable because strength and toughness may be reduced.
  • a fine carbonitride may be formed to secure intended mechanical properties, in particular, excellent strength and toughness even after a PWHT and to secure excellent surface quality as well.
  • the steel material of the present invention secures tensile strength of 480 MPa or more and Charpy impact absorption energy of 150J or more at a temperature of -50°C in a 1/4t point (where t refers to a thickness (mm) of the steel material) even after PWHT, and may secure excellent surface quality in which the number of star cracks in a surface of the steel material is less than or equal to 1 per unit area (50m 2 ) .
  • the steel material of the present invention secures a tensile strength of 480 MPa or more and Charpy impact absorption energy of 100J or more at a temperature of -50°C in a 1/2t point of the steel material after a PWHT.
  • a steel slab, satisfying the alloy composition and Relational Expression 1 proposed in the present invention, is manufactured and then reheated within a certain temperature range.
  • the steel slab may be a continuous casting slab obtained through continuous casting at a temperature of Ar3 or higher, or a forging slab obtained by forging the continuous casting slab to a thickness decreased by 10 to 60% of an initial thickness, but the present invention is not limited thereto.
  • the steel slab is reheated within a temperature range of, in detail, 1050 to 1200°C.
  • the reheating temperature is higher 1200°C, austenite grains may be coarsened, and thus, a target level of physical properties may not be secured.
  • the reheating temperature is less than 1050°C, it may be difficult to re-solid-solubilize a carbonitride formed in the slab.
  • the reheated steel slab is rough-rolled and finish hot-rolled to manufacture a hot-rolled steel sheet.
  • the rough rolling may be performed within a temperature range of, in detail, Ar3+100°C to 1200°C.
  • Ar3+100°C an appropriate temperature may not be secured during a subsequent finish hot rolling.
  • the finish hot rolling is performed at a temperature of, in detail, Ar3+30°C or higher.
  • the temperature is lower than Ar3+30°C during the finish hot rolling, rolling load may make it difficult to perform normal hot rolling, so that a quality defect may occur.
  • the hot rolling is completed as described above, and air cooling is then performed at room temperature.
  • the air-cooled hot-rolled steel sheet is reheated at a temperature of Ac3 or higher, in detail, within a temperature range of Ac3 or higher to 950°C or less, and then maintained at the temperature for (1.3t+20) minutes or more (where t refers to a thickness (mm) of a steel material).
  • the reheating may be performed to form a structure of the hot-rolled steel sheet as a single-phase structure of austenite.
  • a structure of the hot-rolled steel sheet as a single-phase structure of austenite.
  • the temperature is less than Ac3 or the maintaining time is less than (1.3t+20) minutes, a two-phase structure of ferrite and austenite may be formed, so that a final structure may not be obtained as a desired structure.
  • the reheating temperature is higher than 950°C, the austenite structure may be coarsened, so that sufficient impact toughness may not be secured.
  • the hot-rolled steel sheet in which the structure is formed as 100% of an austenite phase by the reheating process, is quenched to room temperature at a cooling rate of 2°C/sec or higher.
  • An upper limit of the cooling rate during the cooling is not necessarily limited, but may be limited to 100°C/sec in consideration of cooling equipment.
  • a tempering process in which the steel sheet is maintained within a temperature range of 600 to 700°C for (1.9t+30) minutes or more, is performed.
  • the steel sheet After the tempering is completed, the steel sheet is air-cooled to room temperature.
  • the steel material of the present invention may be further subjected to a post weld heat treatment (PWHT) process to relieve residual stress caused by a welding process added when the steel material is manufactured as a pressure vessel, or the like.
  • PWHT post weld heat treatment
  • Conditions for the PWHT process are not necessarily limited, but the PWHT may be performed within a temperature range of 595 to 635°C for 120 minutes or more.
  • the steel material of the present invention secures tensile strength of 480 MPa or more and Charpy impact absorption energy of 100 J or more at a temperature of -50°C even after a PWHT.
  • the steel material of the present invention has a thickness of 10 to 150mm.
  • a thickness of the steel material of the present invention is less than 10mm, a shape defect may occur during a cooling process subsequent to a rolling or reheating process.
  • the thickness of the steel material of the present invention is greater than 150mm, a cooling rate may be insufficient during a cooling process subsequent to a reheating process, and thus, a ferrite fraction may be excessively increased to result in difficulty in securing a target level of strength.
  • Steel slabs having alloy compositions listed in Table 1, were reheated within a temperature range of 1120 to 1170°C according to respective thicknesses (mm) thereof, and then finish hot-rolled at a temperature of 930 to 1000°C to prepare hot-rolled steel sheets.
  • Each of the hot-rolled steel sheets was air-cooled to room temperature, reheated to a temperature of 910°C, and then quenched to be cooled to room temperature.
  • Each of the cooled hot-rolled steel sheets were tempered within a temperature range of 635 to 680°C, and then air-cooled to room temperature to manufacture a steel material.
  • Each of the steel materials was subjected to a post weld heat treatment (PWHT) in which each of the steel materials was maintained at a temperature of 615°C for 165 minutes. In this case, the PWHT was performed twice.
  • PWHT post weld heat treatment
  • a specimen was taken in a direction, perpendicular to a rolling direction, and then observed in a 1/4t point with an optical microscope, and then ferrite, bainitic ferrite, and martensite phases were distinguished from each other by naked eye using electron backscatter diffraction (EBSD) equipment, and then a fraction of each of the phases was measured.
  • EBSD electron backscatter diffraction
  • the impact toughness was evaluated by performing a Charpy V-notch impact test at a temperature of -50°C. The impact toughness was measured three times, and an average value and individual values thereof are listed.
  • thickness-dependent microstructures of steel materials may be confirmed. Specifically, it may be confirmed that a steel material was formed to have a mixed structure of ferrite, bainitic ferrite, and tempered martensite when a thickness of the steel material was 30 mm or less, whereas a steel material was formed to have a two-phase structure of ferrite and bainitic ferrite when a thickness of the steel material was greater than 30 mm.
  • Comparative Steels 1 to 3 having alloy compositions which do not satisfying the present invention, were all formed to have a single-phase structure of tempered martensite because a great number of hardenability elements were contained when a thickness of a steel material was 60 mm or less, and were formed to have a composite structure of ferrite and bainitic ferrite or a composite structure of ferrite, bainitic ferrite, and tempered martensite when a thickness of a steel material was greater than 60 mm.
  • Inventive Steels 1 and 2 exhibit excellent surface quality because no star crack was generated in surfaces thereof, whereas Comparative Steels 1 to 3 exhibit deteriorated surface quality because the number of generated star cracks were 5/50m 2 , 4/50m 2 , and 5/50m 2 , respectively (see FIG. 1 ).
  • star cracks are considered to be generated because a grain boundary was embrittled due to enrichment of copper (Cu) included in a large amount.
  • Inventive Steels 1 and 2 had excellent strength and impact toughness in a 1/4t point and a 1/2t point in a thickness direction after a PWHT, whereas Comparative Steels 1 to 3 had deteriorated impact toughness after a PWHT.
  • Comparative Steels 2 and 3 each having a relatively high content of titanium (Ti) as compared with Comparative Steel 1, a coarse (Ti,Nb) (C,N) carbonitride was formed as they were exposed to a high temperature for a lengthy period during a manufacturing process, and impact toughness is considered to be deteriorated because the carbonitride acted as a factor to induce cracking.

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Claims (7)

  1. Stahlmaterial für einen Druckbehälter mit ausgezeichneter Oberflächenqualität und Schlagzähigkeit, wobei das Stahlmaterial
    in Gewichtsprozent (Gew.-%) 0,1 bis 0,15 % Kohlenstoff (C), 0,15 bis 0,5 % Silicium (Si), 1,2 bis 1,8 % Mangan (Mn), 0,01 % oder weniger, mit Ausnahme von 0 %, Phosphor (P), 0,01 % oder weniger, mit Ausnahme von 0 %, Schwefel (S), 0,01 bis 0,05 % Aluminium (Al), 0,01 bis 0,05 % Niob (Nb), 0,01 bis 0,25 % Nickel (Ni), 0,1 % oder weniger (mit Ausnahme von 0 %) Kupfer (Cu), 0,01 bis 0,1 % Molybdän (Mo), 0,01 bis 0,05 % Vanadium (V), 0,003 % oder weniger (mit Ausnahme von 0 %) Titan (Ti), 5 ppm oder weniger, mit Ausnahme von 0 ppm, Bor (B), 20 bis 100 ppm Stickstoff (N) und einen Rest aus Eisen (Fe) und anderen unvermeidlichen Verunreinigungen umfasst und den untenstehenden Beziehungsausdruck 1 erfüllt,
    wobei das Stahlmaterial ein Verbundgefüge aus Ferrit und bainitischem Ferrit als Mikrogefüge umfasst, wobei, wenn eine Dicke des Stahlmaterials größer als 35 mm ist, das Stahlmaterial Ferrit mit einem Flächenanteil von 40 bis 80 % und als Rest bainitischen Restferrit als Mikrogefüge umfasst, und wobei, wenn eine Dicke des Stahlmaterials 35 mm oder weniger beträgt, das Stahlmaterial Ferrit mit einem Flächenanteil von 20 bis 60 %, angelassenen Martensit mit einem Flächenanteil von 20 % oder weniger, einschließlich 0 %, und einen Rest aus bainitischem Restferrit als Mikrogefüge umfasst, wobei an einem Punkt 1/4 t, wobei sich t auf eine Dicke (mm) des Stahlmaterials bezieht, die Zugfestigkeit 480 MPa oder mehr beträgt und die Charpy-Schlagabsorptionsenergie bei einer Temperatur von -50°C 150 J oder mehr beträgt, wobei an einem Punkt 1/2 t, wobei sich t auf eine Dicke (mm) des Stahlmaterials bezieht, die Zugfestigkeit 480 MPa oder mehr beträgt und die Charpy-Schlagabsorptionsenergie bei einer Temperatur von -50°C 100 J oder mehr beträgt, wobei der Charpy-Wert gemäß der Beschreibung bestimmt wurde, wobei das Stahlmaterial eine Dicke von 10 bis 150 mm aufweist, wobei sich Ferrit auf nadelförmigen Ferrit bezieht, 0,5 < Ti + Nb + B / 3,5 N + Cu/Ni < 2,5
    Figure imgb0005
    wobei sich jedes Element auf einen Gehalt in Gew.-% bezieht.
  2. Stahlmaterial nach Anspruch 1, wobei das Stahlmaterial ein M(C,N)-Carbonitrid mit einem durchschnittlichen Korndurchmesser von 100 nm oder weniger in einem Mikrogefüge umfasst, wobei M Niob (Nb), Titan (Ti) oder Vanadium (V) ist und ein Anteil von Nb 20 % oder mehr beträgt, wobei diese Werte gemäß der Beschreibung bestimmt wurden.
  3. Stahlmaterial nach Anspruch 1, wobei die Anzahl sternförmiger Risse in einer Oberfläche des Stahlmaterials weniger als oder gleich 1 pro Flächeneinheit beträgt, wobei die Flächeneinheit 50 m2 ist.
  4. Stahlmaterial nach Anspruch 1, wobei das Stahlmaterial einer Wärmebehandlung nach dem Schweißen (PWHT) unterzogen wird.
  5. Verfahren zur Herstellung eines Stahlmaterials für einen Druckbehälter mit ausgezeichneter Oberflächenqualität und Schlagzähigkeit nach Anspruch 1, wobei das Verfahren Folgendes umfasst:
    Wiedererwärmen einer Stahlbramme auf eine Temperatur von 1050 bis 1200 °C, wobei die Stahlbramme in Gewichtsprozent (Gew.-%) 0,1 bis 0,15 % Kohlenstoff (C), 0,15 bis 0,5 % Silicium (Si), 1,2 bis 1,8 % Mangan (Mn), 0,01 % oder weniger, mit Ausnahme von 0 %, Phosphor (P), 0,01 % oder weniger, mit Ausnahme von 0 %, Schwefel (S), 0,01 bis 0,05 % Aluminium (Al), 0,01 bis 0,05 % Niob (Nb), 0,01 bis 0,25 % Nickel (Ni), 0,1 % oder weniger, mit Ausnahme von 0 %, Kupfer (Cu), 0,01 bis 0,1 % Molybdän (Mo), 0,01 bis 0,05 % Vanadium (V), 0,003 % oder weniger, mit Ausnahme von 0 %, Titan (Ti), 5 ppm oder weniger, mit Ausnahme von 0 %, Bor (B), 20 bis 100 ppm Stickstoff (N) und einen Rest aus Eisen (Fe) und anderen unvermeidlichen Verunreinigungen beinhaltet und den untenstehenden Beziehungsausdruck 1 erfüllt;
    Vorwalzen der wiedererwärmten Bramme in einem Temperaturbereich von Ar3 + 100 °C bis 1200 °C;
    Fertigwarmwalzen der vorgewalzten Bramme bei einer Temperatur von Ar3 + 30 °C oder höher, um ein warmgewalztes Stahlblech zu erzeugen;
    Luftkühlen des warmgewalzten Stahlblechs auf Raumtemperatur;
    Wiedererwärmen des luftgekühlten warmgewalzten Stahlblechs auf eine Temperatur von Ac3 oder höher und anschließendes Halten des warmgewalzten Stahlblechs während (1,3 t + 20) Minuten oder mehr, wobei sich t auf eine Dicke des Stahlmaterials in mm bezieht;
    Abkühlen des warmgewalzten Stahlblechs auf Raumtemperatur mit einer Abkühlungsrate von 2 °C/sec oder mehr nach dem Halten;
    Durchführen eines Anlassprozesses, bei dem das abgekühlte warmgewalzte Stahlblech während (1,9 t + 30) Minuten oder mehr in einem Temperaturbereich von 600 bis 700 °C gehalten wird; und
    Luftkühlen des warmgewalzten Stahlblechs auf Raumtemperatur nach dem Durchführen des Anlassprozesses, 0,5 < Ti + Nb + B / 3,5 N + Cu/Ni < 2,5
    Figure imgb0006
     wobei sich jedes Element auf einen Gehalt in Gew.-% bezieht.
  6. Verfahren nach Anspruch 5, wobei die Bramme eine Stranggussbramme, die durch Stranggießen erhalten wird, oder eine Schmiedebramme ist, die erhalten wird, indem die Stranggussbramme auf eine Dicke geschmiedet wird, die um 10 bis 60 % einer Ausgangsdicke verringert ist.
  7. Verfahren nach Anspruch 5, ferner umfassend:
    Durchführen einer Wärmebehandlung nach dem Schweißen (PWHT) an dem warmgewalzten Stahlblech, das nach dem Durchführen des Anlassprozesses auf Raumtemperatur abgekühlt wurde, in einem Temperaturbereich von 595 bis 635 °C während 120 Minuten oder mehr.
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